Method to lower the release of hazardous air pollutants from Kraft recovery process

ABSTRACT

A method for removal of pollutants from a pulping process byproduct liquor, the method comprising injecting an oxygen-containing gas into said liquor and condensing the water vapor from the stripping gas so as to produce a condensate comprising pollutants.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patent application Ser. No. 09/962,538, filed Sep. 24, 2001, the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention is directed toward the reduction of hazardous air pollutants and reduced sulfur compounds from a byproduct stream produced in a pulping process which contains water where a portion of the water is removed in a multiple effect evaporation system and a further portion of the water is removed in a direct contact evaporator (DCE).

The Kraft pulping process produces cellulose fibers and black liquor. Black liquor is a water based mixture of organic wood derivatives and alkaline pulping chemicals, chiefly containing degraded lignin, organic acid salts, resin, sodium hydroxide, and sodium salts including carbonate, sulfide, sulfate, sulfite, and thiosulfate. Trace compounds in the black liquor include, methanol, benzene, methyl mercaptan and dimethyl disulfide. Weak black liquor contains 15 wt % dissolved and suspended solids of which about 80% are organic and the remainder are inorganic compounds.

Weak black liquor is concentrated to about 45-50 wt % solids by multiple effect evaporation and further evaporated to about 65 wt % solids in a DCE, where the liquor is contacted with the flue gases from the recovery boiler. The concentrated liquor is combusted in a recovery boiler to raise steam and to recover sulfur and sodium for reuse in the pulping step. Oxidation of the sodium hydrosuifide in the black liquor is necessary prior to its introduction into the DCE to minimize the emissions of hydrogen sulfide in the flue gas from the recovery boiler. Newer mills have replaced DCEs with indirectly-heated concentrators, which substantially reduces the total reduced sulfur emissions in the flue gas of the recovery boiler.

Most oxidation processes use air as the oxidant. In these processes the liquor from the multiple effect evaporators is conveyed to an atmospheric reaction vessel where air is sparged through the liquor and a portion of the oxygen in the air reacts with the sulfide in the liquor to produce sodium thiosulphate. Unreacted oxygen, nitrogen, water vapor and volatile compounds, which have been transfered from the black liquor to the gas phase, are typically vented to the atmosphere. The volatile compounds include methanol and benzene, which are considered hazardous air pollutants (HAP), and methyl mercaptans and dimethyl disulfide, which are considered total reduced sulfur (TRS) pollutants. The total amount of HAP and TRS emissions in the vent of the air oxidizer is a function of their concentration in the black liquor entering the oxidation system, the amount of air used in the process and the amount of oxygen that reacts with the black liquor.

After the oxidation step, the liquor is conveyed to the DCE where the liquor contacts the hot flue gases from the recovery boiler. This process humidifies the flue gas and concentrates the black liquor. A portion of the remaining volatile compounds in the black liquor are transferred to the gas phase and discharged to the atmosphere in the flue gas from the DCE. Hydrogen sulphide can also be produced and transported to the gas phase via the reaction NaSH+H₂O

H₂S_((g))+NaOH. The amount of H²S_((g)) produced is a function of the concentration of NaSH in the black liquor. The total amount of HAP and TRS emissions in the flue gas is primarily a function of the concentration of these compounds in the liquor entering the DCE. It was generally believed that the total amount of HAP and TRS emissions from the combination of air oxidizer and DCE were primarily a function of their concentration in the black liquor from the multiple effect evaporators.

An alternative to the air oxidation process is described in U.S. Pat. Nos. 4,239,589, and 4,313,788, which disclose the oxidation of black liquor in which high recovery of the heat of reaction is accomplished by integration with multiple effect evaporator stages. The process uses a gas with a high concentration of oxygen, typically greater than 99%. The oxidized liquor from the evaporators is typically sent to storage and then passed to the DCE. The higher concentration of HAP and TRS in the black liquor entering the DCE as compared to black liquor from an air oxidation system results in a proportionate increase in the transfer of these compounds to the flue gas from the DCE. It is generally believed that the total emissions of HAP and TRS from the DCE in the oxygen systems disclosed in U.S. Pat. Nos. 4,239,589, and 4,313,788 will be approximately equal to the amount emitted from the combination of an air oxidation system and a DCE.

It is a generally desirable to minimize the emission of HAP and TRS compounds into the atmosphere. As a result, Kraft pulp mills consider methods of reducing these emissions. Two identified methods to lower these emissions are the collection and incineration of the vent gas from the air oxidation system or conversion of the DCE recovery boiler to a low odor configuration. Unfortunately both options are expensive.

Improved methods of reducing the total HAP and TRS emissions from the recovery area of a pulp mill are needed. The present disclosure and the claims, which follow, describe such an improved method.

As much as the background of the invention described above and the details of the invention described below relate to the Kraft pulping process, it is to be understood that there are other pulping processes, such as soda pulping that do not use sulfur in the process and therefore do not require the oxidation of a byproduct stream for odor control. However, these processes typically produce a byproduct stream which is subject to concentration in a multiple effect evaporator system and a direct contact evaporation system and in which there are HAPs. In this regard, the present invention can be employed to decrease the HAPs in the byproduct stream fed to the DCE thereby reducing the HAP emissions from the DCE. Therefore this invention can be used in many pulping processes to reduce the HAP emissions from the processes.

BRIEF SUMMARY OF THE INVENTION

In one general aspect, the invention is a pulping process that produces a byproduct liquor comprising the steps of:

-   -   a) determining the relationship between the amount of         oxygen-containing gas injected into a reactor containing the         byproduct liquor or the amount of stripping steam generated in         said reactor and the amount of methanol generated in said         reactor;     -   b) injecting oxygen-containing gas into said reactor containing         the byproduct liquor, such that said injecting results in both a         further concentrated liquor and a stripping gas, said stripping         gas comprising both water vapor and methanol;     -   c) controlling said injection of said oxygen-containing gas so         as to stop said injecting step of said oxygen-containing gas at         a point of required conversion of the sulfides when: (i)         continued injection of said oxygen-containing gas results in         methanol production at a faster rate than it can be removed by         said stripping gas, or (ii) both at a point that is at or after         the point of required conversion of the sulfides and prior to a         point at which said oxygen-containing gas reacts to form         methanol at a faster rate than is removed by said stripping gas;         and     -   d) separating said stripping gas from said further concentrated         liquor.

In another embodiment, the invention comprises the additional step after said step (c), which includes after step (d), of injecting a second oxygen-containing gas into said reactor comprising the byproduct liquor, such that said injecting results in both a further concentrated liquor and a second stripping gas, said second stripping gas comprising both water vapor and methanol.

In another embodiment the invention further comprises the step of:

-   -   e) condensing said water vapor from said stripping gas so as to         produce a condensate comprising said methanol.

In many preferred embodiments, subsequent to step (c) the liquor is processed in a flash tank so as to produce a further concentrated liquor and both water vapor and methanol and wherein said water vapor is condensed to produce a condensate comprising said methanol, and wherein said condensate is separated from said further concentrated liquor. Flashing can occur in a flash tank, a vaporizer, an evaporator, or another vessel that provides a environment, which in some embodiments may be a lower pressure, in which the vapor phase can be separated from the liquor liquid phase. The use of the term “flash tank” includes all of those vessels unless otherwise indicated.

In another embodiment of the invention there is an additional injecting step of a second oxygen-containing gas into the reactor or another reactor that results in the formation of a second stripping gas comprising both water vapor and methanol.

In one preferred embodiment, the oxygen-containing gas in step (b) is the oxygen-richer gas, and at a time prior to step (b), there is a step comprising injecting a first oxygen-containing gas into said concentrated liquor, said first gas being the oxygen-poorer gas by virtue of the fact that it contains a lower percentage of oxygen than the oxygen-richer gas does. The oxygen-poorer gas can be air. These injection steps can be performed in the same or different reactors.

The invention is expressed in terms of methanol. However, it is believed that the concentration of one or more other pollutants in the liquor will be decreased as that of methanol is decreased by removing it and them from the black liquor by injecting one or more oxygen-containing gases or one or more oxygen-containing and non-oxidizing gases into the reactor, or in some embodiments more than one reactor.

The measurement of the percentage of oxygen that reacts can be calculated based on knowledge of the amount of oxygen that is inputted in step (b) and the amount of unreacted oxygen that is expelled as vent gas from the condensor that condenses the flash tank vapor. Oxygen content of a vent gas can be determined by well known techniques. (For example, a Servomex oxygen analyzer which employs the paramagnetic property of oxygen.) Measuring the amount of unreacted oxygen in the vent gas is useful information to determine the extent of oxidation.

Prior to the disclosure of this invention, it was believed that oxygen reacted with sulfides and that any other oxidation reaction(s) created acids and other non-volatile materials that stayed in the black liquor. It was not realized that large quantities of volatile compounds could be created in the black liquor faster than they could be removed via the stripping gas and that the rate of generation depended on the extent of oxidation of the black liquor in the reactor. In order to know what oxidation reactions are taking place in the reactor, a compositional analysis of the black liquor and the vent gas over time can be performed which will be described in more detail below.

In another preferred embodiment, the method further comprises combining, with new byproduct liquor that has not yet been processed through steps (b), (c) and (d) at least a portion of the further concentrated liquor so as create a mixture of said new byproduct liquor and said portion of further concentrated liquor and, subsequent to said combining, processing at least a portion of said mixture through the steps (b), (c) and (d) described for a byproduct liquor.

In another general aspect, the invention further comprises the step of:

-   -   e) combining at least a portion of said further concentrated         liquor with said byproduct liquor either previous to said         byproduct liquor entering, as said byproduct liquor enters, or         while said new liquor is in, said reactor; wherein step (b) is         done at temperatures less than 175° C.

In preferred embodiments, step (b) is done at a temperature less than 175√ C. most preferably at a temperature in the range 120 to 135° C.

In preferred embodiments, the pulping process is one in which the oxidation of the liquor takes place in a pressurized reactor operating at greater than 2.0 bar(g).

In another embodiment, the majority of inert gases and unreacted oxygen in the oxygen-containing gas are separated from the by-product liquor at a point subsequent to reaction of the oxygen at a pressure of greater than 2 bar(g).

The oxygen-containing gas can be air. In some embodiments, the oxygen-containing gas comprises an inert gas at a concentration of 1% to 40% (v/v). In particular embodiments of the invention the oxygen-containing gas is an oxygen-enriched gas that may comprise at least 22% (v/v) or at least 70% (v/v) oxygen (at least a high percentage), where the oxygen-containing gas comprises at least 95% (v/v) oxygen (a very high percentage), or at least 70%, and/or where the percentage of oxygen reacting is at least 90%, and combinations thereof. Similarly, additional particular embodiments are where the liquor contains about 25%-60% solids (w/w) or 40% to 55% solids (w/w).

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a prior art method of black liquor oxidation, concentration and combustion, which utilizes direct contact evaporation prior to the recovery boiler, wherein the oxidation process uses air and the vent gases from the oxidation process are collected, cooled and incinerated in which the method of this invention can be used to reduce the methanol emissions from the process.

FIG. 2 is a schematic flow diagram of a prior art method of black liquor oxidation, concentration and combustion, which utilizes direct contact evaporation prior to the recovery boiler, wherein the oxidation process uses oxygen and the oxidation process is performed within the multiple effect evaporator system in which the method of this invention can be used to reduce the methanol emissions from the process.

FIG. 3 is a schematic flow diagram of a method of black liquor oxidation in which the method of this invention can be used to reduce methanol emissions from the process.

FIG. 4 is a schematic diagram of a variation of the invention illustrated in FIG. 3.

FIG. 5 is a schematic diagram of a variation of the invention.

FIG. 6 is a graph showing three lines: (1) the concentration of the methanol remaining in the black liquor as it is increasingly stripped with stripping gas if the assumption is made that there is no methanol generation in the reactor, (2) the concentration of the methanol produced in the black liquor by reaction while injecting the oxygen-containing gas assuming there is no stripping gas, and (3) the concentration of methanol remaining in the black liquor while injecting the oxygen-containing gas and stripping.

FIG. 7 is a graph showing the concentration of methanol remaining in the black liquor while injecting the oxygen-containing gas and stripping.

DETAILED DESCRIPTION OF THE INVENTION

As stated earlier, oxidation of the sodium hydrosulfide in the black liquor is necessary prior to the black liquor's introduction into the DCE to minimize the emissions of hydrogen sulfide in the flue gas from the recovery boiler. To do this an oxygen-containing gas is injected or sparged into or otherwise contacted with the black liquor (The term injected will be used to mean injected, sparged or otherwise contacted). A portion of the oxygen in the oxygen-containing gas reacts with the sulfide in the liquor to produce sulfate. Unreacted oxygen, other components of the oxygen-containing gas (e.g.nitrogen), water vapor and volatile compounds, which have been transferred from the black liquor to the gas phase form what is referred to herein as a stripping gas. (Steam or other non-oxidative gas injected into the reactor may also be referred to as stripping gas or as a non-oxidative gas.) This invention will be described In terms of methanol although other pollutants will be removed with the methanol.

The oxidation reaction to produce the sulfate is an exothermic reaction that heats the liquor. There are other oxidation reactions occurring in the black liquor that also liberate heat into the black liquor. One reaction that is occurring in the black liquor during the injection of the oxygen-containing gas creates methanol, aspects of which have been discovered by the inventors, and shall be described with reference to FIG. 6 and 7. The discovery of the generation of methanol in the black liquor was based on studies of data from Kraft paper mill reactors and computer simulations which were used to create the lines L2 and L3 shown in FIGS. 6 and 7.

The lines L1, L2, and L3 shown in FIGS. 6 and 7 all relate to the amount of methanol in the oxidized black liquor assuming different conditions in a Kraft paper mill reactor in which an oxygen-containing gas is injected into a reactor and generates a stripping gas (steam with additional components therein).

The term reactor will be used herein to refer to a vessel, tank, pipe or other container in which black liquor oxidation occurs or a vessel, tank, pipe or other container, in which a non-oxidative gas is injected into the liquor for the purpose of generating a stripping gas.

In FIGS. 6 and 7, line L1 is a plot of the methanol that remains in the black liquor while the oxygen-containing gas is injected into the black liquor and a stripping gas is produced removing the methanol from the black liquor. This line is drawn based on the prior art understanding that the amount of methanol in the black liquor is at a maximum when it enters the reactor and decreases with increased addition of oxygen-containing gas which causes the stripping of more and more methanol, and therefore a reduction in the amount of methanol in the black liquor. The line was generated using a thermodynamic model to calculate the vapor to liquor partition of methanol. The model was developed employing the thermodynamic properties of methanol, steam and simulated black liquor. The model simulated the generation of an increasing amount of stripping steam (with a constant amount of liquor) and a decreasing amount of methanol in the black liquor with the injection of increasing amounts of oxygen-containing gas in the reactor. Note that as the amount of stripping steam increases, the amount of methanol in the resulting black liquor decreases. The inverse relationship shows the propensity of methanol to partition to the vapor stream.

L2 is a line representing the amount of methanol generated in a reactor excluding the methanol that was in the black liquor prior to introduction into the reactor and not allowing or providing for the removal of any methanol by stripping gas, meaning that all of the methanol that is generated in the reactor stays in the black liquor. Studies performed by the present inventors of data from reactors having an oxygen-containing gas injected therein have shown that as the oxidation proceeds, methanol is generated. As shown by the shape of L2, the methanol generation data for oxidation systems implies minimal generation of methanol until a certain level of injection of oxygen via the oxygen-containing gas and/or a certain level of oxidation of other components, e.g. sulfides, is reached. Once that certain level of injection and/or oxidation is reached, the methanol generation increases from an insubstantial amount with the increasing level of injection of oxygen-containing gas. (In FIGS. 6 and 7 an increasing level of oxygen-containing gas injected is directly related to an increasing level of stripping steam produced, because as stated earlier the oxygen causes exothermic oxidation reactions which creates the steam (the x axis of the graph), therefore the x axis in FIGS. 6 and 7 represents both the amount of steam produced and the related amount of oxygen containing gas injected.

L3 shows the amount of methanol in the black liquor combining the effects of the stripping gas shown in L80%i and the generation of methanol shown in L2. The relationship was derived by combining the relationships shown in L1 and L2 in a computer simulation. First, the amount of methanol generated was calculated based a given level of oxidation. The amount generated was added to the initial amount in the un-oxidized liquor. Flash calculations were then performed on the mixture to the same pressure and temperature conditions as the relationship shown in L1 using the same amount of stripping steam generated for a given amount of oxidation. The flash calculations were repeated for the various levels of oxidation to produce L3.

L3 shows that a minimum is reached in the amount of methanol remaining in the oxidized and flashed black liquor with respect to the level of oxidation. The amount of methanol in the oxidized black liquor that exits the reactor and may subsequently be sent to the DCE is directly related to the amount of methanol generated in the reactor and stripped in the stripping gas. By minimizing the methanol contained in the oxidized black liquor, overall methanol emissions are minimized in the DCE and/or other components downstream of the reactor. The amount of methanol that is stripped from the black liquor into the stripping gas can be further treated before any venting to the atmosphere. By effectively treating the stripping gas from the reactor, the overall amount of methanol (HAPs) emissions from the mill can be significantly reduced.

As shown in FIG. 7, the most effective way to reduce the methanol in the black liquor would be to operate the reactor and add just enough oxygen-containing gas to the reactor (creating stripping gas) so that the amount of methanol in the black liquor would move from point A (which is the methanol content of the black liquor in the reactor prior to injecting the oxygen-containing gas) to point C on L3. At point C, if the amount of methanol in the black liquor were still too high for subsequent removal equipment or emission to the atmosphere downstream of the reactor, additional non-oxidative gas, such as steam could be injected into the reactor to strip the methanol (via a second stripping gas) from the black liquor. That option and the resulting amount of methanol in the black liquor are represented by the line, L4. Alternatively, adding oxygen-containing gas at point C would result in the generation of methanol and an increase in the methanol content of the black liquor, and the resulting amount of methanol would increase along L3 from point C toward and/or past point E, depending upon how much oxygen-containing gas was injected.

Although it would be beneficial for the reduction of the amount of methanol (HAP) in the black liquor to inject the required amount of oxygen-containing gas to operate the reactor to point C, it may be more preferable or required by environmental regulations to add an amount of oxygen-containing gas so as to provide for a required or desired amount of oxidation of the sulfides present in the black liquor. Stated differently, the extent of oxidation of the black liquor is typically driven by the TRS emission limits for the boiler stack gases. Therefore, the amount of oxygen-containing gas required or desired to be injected into the reactor for the oxidation of the sulfides may result in an amount of methanol in the black liquor that is not at point C on L3, and may be either be at a point prior to the minimum at point C, e.g. at point B, or at a point that is past the minimum, e.g. at point E on L3.

If the reactor operates, such that the required amount of oxygen-containing gas injected into the reactor and the amount of methanol in the black liquor is prior to the minimum, e.g. at point B, the methanol in the black liquor may be decreased by additional stripping. Point B is a point prior to the generation of significant amounts of methanol by oxidation of the black liquor. At point B, additional stripping can be provided by: (i) injecting more oxygen-containing gas into the reactor, or (ii) injecting a non-oxidative gas into the reactor, such as steam, to remove additional methanol from the black liquor or (iii) a combination of (i) and (ii). By any of the injecting steps (i), (ii) or (iii) the amount of methanol in the black liquor would move along L3 from point B towards point C; however, in the case of injecting oxygen-containing gas, care should be taken to stop the addition of oxygen-containing gas prior to or at point C to prevent the increase in the methanol content in the black liquor by oxidation reactions that generate methanol and increase the methanol content of the black liquor past point C towards and/or past point E on L3. In this embodiment in which point B represents the amount of oxygen-containing gas sufficient to react with the sulfides, at point C or at point B (as described by option (ii)), a non-oxidative gas can be injected into reactor which will strip the methanol from the black liquor towards or past point D on L4 depending on how much non-oxidative gas is injected. (If the addition of non-oxidative gas (e.g. steam) begins at point B on L3, the amount of methanol in the reactor will pass through point C on L3 before following line L4. Once on L4, the amount of methanol in the further concentrated black liquor decreases by adding increasing amounts of non-oxidative gas. The non-oxidative gas may be from an external source. The non-oxidative gas that is injected into the reactor after or simultaneously with the injection of the oxygen-containing gas results in the formation of a second stripping gas (the first stripping gas being the stripping gas generated by the exothermic reactions caused by the injection of the oxygen-containing gas into the reactor). The non-oxidative gas may be steam or other gas from a source different from the oxygen-containing gas. The non-oxidative gas may be nitrogen or other inert gas. The first and second stripping gases may be removed from the black liquor after injection into the black liquor serially or simultaneously depending on the process. Additionally, one or more reactors may be used to generate the first and second stripping gases.

In another embodiment, the amount of oxygen-containing gas that is required to be injected into the reactor to reduce the TRS emissions may be such that the methanol content of the further concentrated black liquor is at point E on L3. At point E, any additional injection of oxygen-containing gas will only increase the methanol in the further concentrated black liquor; therefore, it is desirable to stop the addition of the oxygen-containing gas as soon as the required conversion of the sulfides is complete. From point E, if any additional methanol reduction is desired or required, the use of a non-oxidative gas to form a second stripping gas, e.g. steam, will result in a reduction of the methanol in the further concentrated black liquor in accordance with L5 in the direction from point E to point F.

Prior to this invention, the relationship between methanol generated in the reactor and the oxidation of the liquor was not understood. It was believed in the prior art that the best way to ensure that all the TRS emissions were minimized was to over-oxidize the further concentrated black liquor. This invention has shown that over-oxidizing the further concentrated black liquor may cause an increase in the methanol in the further concentrated black liquor. This problem had not been recognized. To prevent the over-oxidation of the further concentrated black liquor for a paper mill, the first step is to study the mill and the conditions of the reactor to generate data that indicates the relationship of the concentration of the methanol in the further concentrated black liquor versus the amount of stripping gas generated in the reactor and/or the amount of oxygen-containing gas injected into the reactor or the extent of oxidation of the liquor in order to generate a line similar to L3 for the process being studied. If desired and possible for a given process, the reaction conditions can be modified in the reactor to change the onset of methanol production, and favor the oxidation reaction with sulfides; thereby modifying the curve L3. Such modification may involve using an oxygen-containing gas with a higher concentration of oxygen than air or adjusting the concentration of the oxygen in the oxygen-containing gas. Additional modifications include modifying reaction conditions, such as, temperature and pressure, solids concentration, and reactor design.

Once L3 has been created for a given reactor and process, the necessary amount of stripping steam generated or the necessary amount of injected oxygen-containing gas to provide the required oxidation reaction of the sulfides is determined. Once the required amount of stripping steam created by the required level of oxidation to react with the sulfides is determined, then the amount of methanol in the further concentrated black liquor is known and the location of the point on L3 is known. Once it is known on which side of the minimum quantity of methanol (point C) the process is operating after receiving the required amount of oxygen-containing gas, then the appropriate process steps to control the amount of methanol in the further concentrated black liquor can be made. As discussed above, if the process is known to operate at a high level of oxidation past the minimum point C on L3, due to stringent TRS emission permitted levels, then stripping by injecting a second gas is preferred. However, if the process is known to operate at a low level of oxidation before the minimum point on L3, oxygen-containing gas can continue to be used to generate the stripping steam and thereby lower the methanol in the further concentrated black liquor.

The lines L1, L2 and L3 were generated using computer modeling and actual data; however, lines similar to L1 have been in the prior art. L3, the relationship between methanol production and the amount of black liquor oxidation, for a particular paper mill can be generated by measuring the methanol content in the further concentrated black liquor in the reactor as a function of the stripping gas generated (or oxygen-containing gas and/or non-oxidative gas injected into the reactor(s)). The methanol content in the reactor can be measured by taking samples and performing methanol assays from the reactor over time, or by installing real time methanol measuring equipment which can continuously monitor the amount of methanol in the byproduct liquor and/or further concentrated liquor and/or the stripping gas. Alternatively, L3 can be generated by performing laboratory simulations of the reactor using actual or simulated byproduct liquor. For liquor systems using more than reactor or more than one oxidizing injected gas steps into the reactor or reactors, L3 relationships need to be determined for each reactor and for each of the oxidizing gas streams injected into the reactor or reactors. In a preferred method of this invention after the process steps have been implemented for a particular reactor, monitoring equipment could be installed where possible to measure the quantity of methanol in the stripping gas or gases and in the further concentrated black liquor to determine if the proper amount of oxygen-containing gas(es) and/or non-oxidative gas(es) has been injected into the reactor or reactors, and to be sure the reactor(s) is (are) working properly.

The invention is an improved method of reducing the emissions of HAP and TRS from the recovery area of a pulp mill, and is useful in systems in which air and/or oxygen rich gas compositions are used for black liquor oxidations in a reactor. The black liquor, which may have been processed through the multiple effect evaporators, is contacted with an oxygen-containing gas, which may be air or may be an alternative gas having an oxygen concentration of the gas of at least 22%. In some embodiments, at least 40% of the oxygen reacts with compounds in the black liquor. In those embodiments as described above the heat of reaction produces a stripping gas of sufficient temperature and volume to transfer a substantial portion of the HAP and TRS compounds from the black liquor to the gas phase. Optionally, a flash tank or a series of flash tanks optionally at decreasing pressures can be used in the process at a point subsequent to the reactor so as to produce the majority of the stripping gas. The amount of stripping gas and hence the amount of HAP and TRS transferred to the gas phase is controlled by varying the amount of oxygen-containing gas fed to the system and/or the concentration of oxygen in the oxygen-containing gas, and the amount of a second gas injected into the reactor or additional reactor(s) to form a second stripping gas, if used.

After the reactor, the stripping gas (or gases) is treated. One way to treat it is to cool the stripping gas, whereupon the majority of the water vapor is condensed. When the gas phase is treated this way, a large portion of the HAP, primarily methanol, and a portion of the TRS compounds are transferred to the condensed water vapor (i.e., condensate). The remaining low volume gas stream of the stripping gas may be discharged to the atmosphere or conveyed to a combustion source for incineration.

The improvement of the present invention can be used in any recovery system of a pulp plant that employs black liquor oxidation, for example, those shown in FIG. 1-5. Referring to FIG. 1, weak black liquor 1 from the wood pulping and washing steps is fed to multiple effect evaporation system 101. Weak black liquor contains water, dissolved lignin and other wood constituents, sodium salts (particularly sodium sulfide and other unoxidized sulfur compounds), and sodium hydroxide. Trace compounds in the black liquor include HAP such as, methanol, acetaldehyde and benzene, and TRS such as methyl mercaptan, dimethyl sulfide and dimethyl disulfide. The weak black liquor typically contains 15 wt % solids. The liquor is concentrated in multiple effect evaporator system 101 heated by steam 3 as is known in the art to yield partially concentrated black liquor 5 and residual steam/condensate 7. A portion of the HAP and TRS compounds in the weak black liquor are transferred to the steam/condensate 7 and a portion remain in the partially concentrated black liquor 5.

The partially concentrated black liquor 5, typically at 90° C. and containing typically 48 wt % solids flows to air black liquor oxidation system (reactor) 103 in which air 9 oxidizes the sodium hydrosulfide to sodium thiosulphate via the reaction: 2NaSH+2O₂

Na₂S₂O₃+H₂O and a portion of the organic material in the black liquor.

The amount of air sparged into the liquor is controlled so as to convert at least 90% of the sodium hydrosulfide to sodium thiosulphate. The percent of oxygen in the air 9 that specifically reacts with black liquor is typically no greater than 40%. To achieve a NaSH concentration of less than 2 g/l in the black liquor a significant excess of air 9 is required and the percent of oxygen in the air that reacts with the black liquor is generally no greater than 30%.

Unreacted oxygen, the nitrogen in the air, water vapor, and HAP and TRS compounds are vented 11 from the reactor to a heat transfer device 109, herein called a condensor, where the gas 11 is cooled and a portion of the water vapor and HAP and TRS are condensed and discharged from the condensor in stream 32. The cooled vent gas 28 flows to a combustion device 111 where the HAP and TRS compounds are incinerated and the products of combustion 29 are discharged to the atmosphere. The concentration of volatile compounds, including methanol, benzene, acetaldehyde, acetone, methyl mercaptans, and dimethyl disulfide, in the vent gas 11 is a function of thermodynamic equilibrium. The total amount of these compounds in the vent gas 11 is dependent upon their concentration in the feed black liquor 5, the amount of air fed 9 to the oxidizer system, the temperature of the feed black liquors and the amount of oxygen reacted. Further, in accordance with this invention it is now understood that the concentration of methanol and possibly other volatile compounds in the reactor 103 and subsequent to the reactor 103 is also a function of the amount of volatile compounds formed in the reactor based on the relationship shown in FIGS. 6 and 7 as explained above.

Oxidized black liquor 13, now containing typically less than 2 g/l of sodium sulfide, and a lower concentration of HAP and TRS compounds, passes to the direct contact evaporation system 105 in which the black liquor is further concentrated by direct contact with hot flue gas 15 from recovery boiler 107. Fully concentrated black liquor 17 and final flue gas 19 flow from the evaporator system 105. Flue gas 19 contains, water vapor, combustion products, and HAP and TRS compounds. The concentration of HAP and TRS compounds in the flue gas 19 is a function of thermodynamic equilibrium. The total amount of these compounds in the flue gas 19 is dependent primarily upon their concentration in the feed black liquor 13.

The black liquor at this point typically contains 65 wt % solids at 115° C. Fully concentrated black liquor 17 is combined with sodium sulfate (salt cake) makeup 20 and passes into recovery boiler 107 in which the organic materials are combusted with air to generate heat withdrawn as steam 21 for use elsewhere in the mill. The inorganic sulfur, largely sodium thiosulfate is reduced to sodium sulfide in the boiler and smelt 23, containing molten sodium sulfide and sodium carbonate, is withdrawn for preparing green liquor. Flue gas streams 27 and 19 pass to a cleanup system for particulate removal typically an electrostatic precipitator. The oxidation of sodium sulfide in black liquor oxidation system 103 required in order to reduce the amount of hydrogen sulfide (another TRS compound) formed in DCE 105 and carried therefrom to the atmosphere in final flue gas 19.

The present invention can be used in the recovery process described in U.S. Pat. Nos. 4,239,589 and 4,313,788, incorporated herein by reference, one embodiment of which is shown in FIG. 2. FIG. 2 is similar to FIG. 1 described above in which weak black liquor 1 from the wood pulping and washing steps is fed to multiple effect evaporation system 101. In FIG. 2, a liquor stream 47 is withdrawn from an intermediate location in the multiple effect evaporators 101. The withdrawn liquor 47, typically at 110° C. and 2.0 bar(g) containing 35 wt % solids flows to a reactor 113 where a gas stream 51 containing typically 99% oxygen, oxidizes the sodium hydrosulfide. The oxidized liquor 49, from the reactor which is now at an elevated temperature, is returned to the multiple effect evaporators 101 where the sensible heat in the liquor is converted to latent heat in the effect receiving the heated black liquor. The liquor proceeds through the remaining effects of the multiple effect evaporators 101 and is discharged as partially concentrated and oxidized black liquor 13, typically at 100° C. and 1.0 bar(g) containing 48 wt % solids flows. Stream 44 is the vapor-side of the multiple effect evaporators. The fully concentrated black liquor 17 proceeds in the same manner as described in FIG. 1.

Another recovery system that the method of this invention can be used in is illustrated in FIG. 3. Partially concentrated black liquor 5, which has been processed through the multiple effect evaporators 101, is passed to a reactor 113 where the liquor is pressurized and an oxygen containing gas 33 having a concentration that may be 95% is injected into the stream. Greater than 90% of the oxygen may be made to react with the black liquor causing an increase in the sensible heat of the black liquor. The oxidized liquor 49 is then introduced into flashtank 117 where the sensible heat is converted to latent heat in the form of water vapor. A portion of the HAP and TRS compounds in the liquor are transferred to the gas phase due to thermodynamic equilibrium. The vent gas 45 is conveyed to a condensor 109 where the gas is cooled and the majority of the water vapor is condensed and removed in condensate stream 32. Additionally, the majority of the methanol and some of the HAP and TRS are condensed and discharged in the condensate stream 32. The vent gas exiting the condensor 28 is discharged to atmosphere or conveyed to an incineration device 111 or discharged into either the high volume, high concentration non-condensible gas system (HVLC-NCG) system or the low volume, high concentration non-condensible gas system (LVHC-NCG). The desired total amount of HAP and TRS transferred to the vent gas is controlled through the reaction of a controlled amount of oxygen 33 with the black liquor 5, so that the oxidation of the sulfides occurs, but that the black liquor is not over-oxidated causing the generation of methanol at a faster rate than can be stripped with the stripping gas. Further a second non-oxidizing gas may be injected into the reactor to further reduce the amount of methanol in the black liquor. Oxidized black liquor 13, now containing typically less than 2 g/l of sodium sulfide, and a controlled concentration of HAP and TRS compounds, passes to the direct contact evaporation system 105 same as described above for FIG. 1. The fully concentrated black liquor 17 proceeds in the same manner as described in the prior art.

An improvement of the present invention over the prior art comprises the ability to control the creation of and transfer of HAP and TRS into the vent gas 45 by controlling the amount of oxygen in the injected oxygen-containing gas 33 reacting with the black liquor 5, and the injection of a second gas to form a second stripping gas if needed to decrease the HAP and TRS in the vent stream from the reactor (not shown). An additional improvement of the present invention is the recovery of the energy in the vent gas 45 at a temperature that is sufficiently high to be of value to the mill when an oxygen enriched gas is used, e.g. one that contains greater than 70% (v/v) O2. In the presented embodiment, the flashtank 117 is operated at 1.0 bar(g). At this pressure approximately 98% of the energy in the vent gas is recovered at a temperature of approximately 90° C., thereby providing energy at a temperature that is of value to the mill. Alternatively, should a mill desire to recover the energy at a higher temperature, the reactor and/or the flashtank can be made to operate at a higher pressure. This option is unavailable in an air oxidation system as the cost to operate the system at pressure would be prohibitively high.

Another embodiment of the invention is shown in FIG. 4 in which oxygen based and air based black liquor oxidation can be used to maximize the stripping of HAP's and TRS from the black liquor. To achieve a low Na2S concentration a mill may have 2 oxidizers (reactors), a primary and a secondary. The primary reactor typically converts about 90% of the Na2S and the percent of oxygen fed to the system, which reacts with the liquor, is high (i.e., 35%). In contrast, the amount of feed oxygen reacting with the liquor in the ‘secondary’ reactor is typically quite low. This means that the temperature of the vent stream of the secondary reactor is lower than the feed temperature of the black liquor (i.e., the energy needed to humidify the vent is greater than the energy released in the oxidation reaction). Therefore, the ability to strip HAPs from the liquor into the vent of the secondary reactor is low (relatively).

Replacing the secondary reactor with an oxygen reactor is illustrated in FIG. 4. One uses a certain amount of air (as the primary oxidant) in a primary air reactor 127, and then replaces the second reactor with an oxygen-based reactor 113, which is connected to the reactor by line 50. In this way, the total amount of vent gas 45 would be less than an oxidation system employing an air secondary reactor. If one opts for the above, the vent flow will be a function of the amount of ‘air’ oxidation employed.

In an alternative embodiment, the air 36 in FIG. 4 can be enriched with an oxygen-containing gas. In another embodiment both reactors can be oxygen-based reactors meaning the reactors are injected with enriched oxygen gas (>22% O2 (V/V)).

In another alternative embodiment, the air reactor and the oxygen reactor are combined into one reactor.

In all the embodiments, it may be preferred to operate at an oxygen concentration such that the nitrogen (argon or other inert gas) that passes through the system and finally into the vent from the condenser is in an amount sufficient to ensure that the concentration of combustible non-condensable gas (NCG) (primarily methyl mercaptans and dimethyl disulfide) is well below the lower flammability limit. In this case, the inert gas dilutes the remaining NCG's to levels below the lower flammability limit (LFL). If this is not done, a separate air or an inert gas stream may be added to the condenser to control the concentration of NCG to below the LFL.

The oxidation reactions are exothermic, and cause a rise in liquor temperature. As the temperature increases, the amount of oxygen reacting with the organic material increases and selectivity to NaSH decreases with increased temperature80%. Also, the solubility of calcium carbonate decreases with temperature. Additionally, from an energy recovery, and environmental perspectives, it is best to conduct the oxidation at a solid concentration of between 40 and 60% solids. However, the NaSH concentration at these concentrations is high, at approximately 35 grams/liter. Therefore the temperature rise through the reactor is high. The higher temperature lowers the selectivity which in turn increases the amount of oxygen needed for NaSH conversion, which increases the temperature. A process has been developed wherein a portion of the black liquor is recycled and cooled (by flashing) to control the temperature of the oxidation reaction.

To address the foregoing, an oxidation system can be designed which allows for temperature control. The system is illustrated in FIG. 5 for a system using a DCE 105. A portion of the liquor is recycled using line 121, which is otherwise the same as FIG. 3. In FIG. 5, the liquor is cooled by flashing in the flash tank 117 and then reintroduced into the reactor 113.

The advantages of the system include: control of the reaction selectivity; a means of ensuring that the solubility limit of calcium carbonate in the black liquor is greater than the actual concentration of the calcium carbonate in the black liquor; and constant reactor operating conditions.

For the embodiments shown in FIG. 4 and FIG. 5, the relationship between the amount of oxidizing gas and methanol generation will have to be determined for each process and taking into account each reactor (for the embodiment shown in FIG. 4) and the effect of the recycle stream (for the embodiment shown in FIG. 5) when determining how much stripping gas to generate (and how much gas to inject into the reactors).

Although illustrated and described herein with reference to certain specific embodiments, the present invention is not intended to be limited to the detailed embodiments shown. One skilled in the art can understand the invention and make various modifications thereto without departing from the basic spirit thereof, and without departing from the scope of the claims which follow. 

1. A method useful in a pulping process that produces a byproduct liquor, said method comprising the steps of: a) determining the relationship between the amount of oxygen-containing gas injected into a reactor containing the byproduct liquor or the amount of stripping steam generated in said reactor and the amount of methanol generated in said reactor; b) injecting oxygen-containing gas into said reactor comprising the byproduct liquor, such that said injecting results in both a further concentrated liquor and a stripping gas, said stripping gas comprising both water vapor and methanol; c) stopping said injecting of said oxygen-containing gas at a point of required conversion of the sulfides when: (i) continued injection of said oxygen-containing gas results in methanol production at a faster rate than it can be removed by said stripping gas, or (ii) both at a point that is at or after the point of required conversion of the sulfides and prior to a point at which said oxygen-containing gas reacts to form methanol at a faster rate than is removed by said stripping gas; and d) separating said stripping gas from said further concentrated liquor.
 2. The method according to claim 1 further comprising: e) condensing said water vapor from said stripping gas so as to produce a condensate comprising said methanol.
 3. The method according to claim 1 wherein after said step (c) is the additional step of injecting a second oxygen-containing gas into said reactor comprising the byproduct liquor, such that said injecting results in both a further concentrated liquor and a second stripping gas, said second stripping gas comprising both water vapor and methanol.
 4. The method according to claim 1 wherein after said step (c) is the additional step of injecting a non-oxidative gas into said reactor comprising the byproduct liquor, such that said injecting results in both a further concentrated liquor and a second stripping gas, said second stripping gas comprising both water vapor and methanol.
 5. The method according to claim 1 wherein after step (d), steps (a), (b), (c) and (d) are repeated for a second reactor downstream of said first reactor into which said further concentrated liquor is fed.
 6. The method according to claim 5 wherein for said repeated steps (a), (b), and (c) for said second reactor, a second oxygen-containing gas is injected.
 7. The method according to claim 1 further comprising the steps of: e) feeding said further concentrated liquor into a second reactor; and f) injecting a non-oxidative gas into said second reactor.
 8. The method according to claim 4 wherein said non-oxidative gas is selected from the group of steam and an inert gas.
 9. The method of claim 1 wherein said oxygen-containing gas is air.
 10. The method of claim 1 wherein said oxygen-containing gas comprises at least 70% (v/v) oxygen.
 11. The method of claim 1 wherein the oxygen-containing gas comprises an inert gas at a concentration of 1% to 40% v/v.
 12. The method of claim 1 wherein subsequent to step (c) the liquor is processed in a flash tank so as to produce a further concentrated liquor and both water vapor and methanol.
 13. A method of claim 1 wherein the oxygen-containing gas in step (b) is the oxygen-richer gas, and wherein at a time prior to step (b), there is a step comprising injecting an oxygen-containing gas into said concentrated liquor, said gas being the oxygen-poorer gas by virtue of the fact that it contains a lower percentage of oxygen than the oxygen-richer gas does.
 14. A method of claim 13 wherein the oxygen-poorer gas is air.
 15. A method of claim 1 which further comprises combining, with new byproduct liquor that has not yet been processed through steps (b) and (c), at least a portion of the further concentrated liquor so as create a mixture of said new byproduct liquor and said portion of further concentrated liquor and, subsequent to said combining, processing at least a portion of said mixture through the steps (b) (c) and (d).
 16. A method of claim 1 wherein step (d) occurs at a pressure of greater than 2 bar(g).
 17. A method of claim 1 which further comprises combining, with new byproduct liquor that has not yet been processed through steps (b) and (c), at least a portion of the further concentrated liquor so as create a mixture of said new byproduct liquor and said portion of further concentrated liquor, and processing at least a portion of said mixture through the steps (b), (c) and (d) described for a byproduct liquor, wherein step (b) is done at temperatures less than 175° C.
 18. The method of claim 17, wherein subsequent to step (d) occurs in a flash tank so as to create a cooled liquor.
 19. A method of claim 17 wherein step (b) is done at a temperature less than 150° C. 20 A method of claim 17 wherein step (b) takes place in a pressurized reactor operating at greater than 2.0 bar(g). 